Issue

Flux Inspection with UV Fluorescence AOI

09/01/2004

Making the Invisible Visible

By GEORGE T. AYOUB

Flux inspection has been a challenge for flip chip and ball grid array (BGA) assemblers because of the inability of inspection systems, including automated optical inspection (AOI), to accurately see the material and inspect it while maintaining line speed. Specifically with respect to flip chips, the inspection of flux is an important part of controlling the process and can prevent costly mistakes. A machine vision solution using ultraviolet (UV) illumination is able to detect defects from flux deposits. This technique replaces the visible AOI light with specialized UV light that matches the properties of the substrate and flux to achieve optimized inspection results.

Importance of Inspecting Flux in the BGA/CSP Assembly

Flux plays a critical role in the process dynamics of BGA/chip scale package (CSP) package assembly.1 A vast range of defects in final assembly can be traced back to poor flux or paste deposition. For example, some of the defects in the final assembly derive from poor flux alignment with respect to the intended pads, insufficient thickness/amount of the flux material, excessive amount of flux, or from smearing. The detection of these pass/fail types of defects (attribute data) at an early stage of the process reduces the assembly cost significantly. Moreover, many manufacturers would agree that it is important to control the process of flux deposition by means of relevant measured variables to detect trends and prevent defects from occurring. This requires a system that is able to measure the key variables of the process (variable data). By providing real-time information on key process parameters, manufacturers can take corrective action and prevent scrap and production loss.

Technology Challenges: Making the Invisible Visible

The process of flux inspection flux has been a challenge to AOI manufacturers because of the inability of visible light to image the flux material well and inspect it. Both high and low angle illuminations in the visible suffer from poor signal-to-noise ratio of the flux, with respect to the background. When the flux is illuminated with UV lighting, however, it fluoresces in the visible and the signal can be captured by means of proper filters designed to eliminate any background light not emanating from the fluorescent flux. Under these conditions, the signal-to-noise ratio between the flux and the background is significantly enhanced. The key to obtaining a good signal-to-noise ratio is the proper design of filters and illumination, which are proprietary, that are adaptable to the flux itself and to the background material (ceramic and possibly FR4) and at the same time able to eliminate the visible background light (Figure 1).

Figure 1. Images under visible light vs. UV light. On the left, this image of parts using standard white light, flux is not visible. On the right, shown with proprietary UV light, flux is visible.

Click here to enlarge image

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Challenge: Speed and Resolution for In-line Systems

Image acquisition parameters play an important part in the capability of the system. Two important parameters are the speed of inspection and the proper optical magnification (resolution). Both are related because speed of acquisition is inversely proportional to the number of pixels acquired, which in turn varies linearly with the square of the magnification. Moreover, illuminating small areas require a large amount of light and an adequate number of pixels on target. The requirements to keep up with very fast cycle times coupled with the high resolution were met by using multiple cameras heads, proper illumination using UV diodes and specialized electronics. Multiple camera heads (in this case three are used) extend the field of view from square to rectangular shape, while at the same time not sacrificing the resolution. The specialized electronics allow the camera to acquire in parallel and matches their speed to the processor computing speed. Using UV diodes assures longevity and stability of the system over time, which is extremely important when an inspection program need to run without modification on different systems or production lines.

Defect Detection, Measurements, Variables and SPC

Using UV fluorescence techniques, the system in operation goes beyond the detection of the pass/fail defects attributes, and assists in enhancing yield by means of SPC techniques on measured variables. It measures the position, area, pad area coverage, and brightness of the deposited flux. Brightness is measured by calculating the median gray scale of the paste blob. Although it is ideal to use the height and volume of the flux, these measurements with the UV technique used here are seen to be dependent on the properties of the flux and the background where the flux is being deposited and cannot be trusted in all cases as absolute measurements. There are good logical reasons backed by experiments confirming that brightness correlates with the height of the flux. In effect, the brightness depends on the amount of fluorescent material in the flux and therefore should vary linearly with the volume. This correlation is not always certain, however, but depends on the environment. Therefore, care should be taken when interpreting the measured brightness since other materials may fluoresce also and add to the noise. The method described has proven to be effective in the production environment, utilizing brightness along with position, area and pad area coverage as measurement parameters, thus providing a logical and adequate means for controlling the final quality of the process by means of SPC methods.

In the production environment, real- time process control has proven to add value to the process by following trends and preventing defects from happening and has been an integral and critical part of the system (Figure 2). Depending on the alarm setting, the system is able to stop the line or turn on a yellow or red light for visual feedback to the operator.

Conclusion

The technique described in this article has been tested with more than three years of in-line inspection. The system is process capable with GRR in the range of 2.5 to 8%. It is able to keep up with a relatively fast production line speed, while achieving a low false call rate. By containing defects at this early stage and controlling the trends with SPC, good results have been achieved. Future planned work is to keep enhancing the signal-to-noise ratio and to extend the application of this technique to different substrates and flux types.